Final answer:
The song of Kazi Nazrul Islam that was declared as the battle song of Bangladesh is "Bidrohi".
Step-by-step explanation:
The song of Kazi Nazrul Islam that was declared as the battle song of Bangladesh is "Bidrohi". This song, also known as "The Rebel" or "The Revolutionary", was written in 1921 and became a symbol of resistance and patriotism during the Bangladesh Liberation War in 1971.